Offshore oil drilling has become a critical factor in supplying present day energy requirements. Offshore drilling is a relatively recent development and has seen great advances in recent years. One highly regarded technique for offshore drilling employs the use of a floating drilling platform which floats on the sea surface. A marine riser extends from the drilling site on the sea floor to a position near the surface. The riser is then connected to the floating platform by a series of connecting lines or cables.
These connecting lines must provide the force necessary to support the marine riser in a near vertical orientation. However, the riser essentially remains fixed relative to the sea floor while the floating platform will rise and fall and move horizontally under the influence of the movement of the ocean surface. Some provision must be made to permit these connecting lines to compensate for the relative motion between the marine riser and the floating platform while maintaining the necessary force to support the marine riser.
In the past, hydropneumatic systems have been mounted on the floating platform and used to pay out or take in the connecting lines while maintaining the necessary tension in the lines. The hydropneumatic systems operate by connecting the line to a piston moving within a cylinder. The piston is permitted to travel the length of the cylinder, while maintaining a predetermined hydraulic or pneumatic pressure on the piston to tension the line.
All of the previous hydropneumatic systems have been active systems. This means that there must be a constant movement of pressurized air or hydraulic fluid as the floating platform moves relative the riser to maintain the system in operation. Therefore, a malfunction of the mechanisms providing for the transfer of the pressurized fluids can seriously compromise the integrity of the attachment of the marine riser to the offshore platform. The use of a piston and cylinder also requires use of seals which can wear and fail in service, resulting in extensive down time and expensive repair. The requirements for pressurized air or fluid also necessitates pumps and associated equipment be mounted on the floating platform, adding to the crowding already present on the platform.
Therefore, a need exists for an improved system capable of providing the desired constant tension in the lines connecting a marine riser to a floating platform and maintaining this tension as the line is payed out or taken in from the platform due to motion of the platform relative to the riser.